A base station that is installed inside user's house of a cellular communication service (mobile phone service) is usually called “femto base station”, “femto-cell base station”, or “home base station”. Further, a small-scale cell formed by a femto base station is called “femto cell”. In contrast to this, a cell formed by a base station that covers a wide area ranging from several hundred meters to several dozen kilometers in a cellular communication service is called “macro cell”. In a service using femto base stations, when a user using a mobile phone is out of his/her house, the mobile phone communicates with a base station forming a macro cell. When the user has returned to his/her house, the mobile phone communicates with a femto base station in a femto cell formed within the house. Note that a cellular communication service usually provides a mobility function. Therefore, even when a mobile phone user is traveling, the user can continue the communication without interruption by changing the base station with which the mobile phone communicates. Similarly, the mobile phone has such a mobility function that even when the user moves from a macro cell to a femto cell, the mobile phone can change the base station with which the mobile phone communicates to the femto base station without interruption.
Note that, in general, a mobile communication system used for a cellular communication service is composed of, at least, a mobile device, a plurality of radio base stations, a base station control station that controls the plurality of base stations, and a core network apparatus that is wire-connected to the base stations control station. The mobility function implements the following two functions depending on the form of transfer of the mobile communication system. A first function is the so-called “soft handover”. In this soft handover, when a mobile device moves from the cell of one radio base station to the cell of another radio base station and these two radio base stations are managed by the same base station control station, the handover is carried out by changing only the radio base station without changing the base station control station. Further, even when two base stations are managed by different base station control stations, the base station control station that has been controlling the base station at the original place can manage the base station at the destination place by connecting the base station control station controlling the original base station with the base station control station controlling the destination base station by using a subscriber line extension technique, and thus requiring substantially no change of the base station control station.
Meanwhile, a second function is the so-called “hard handover”. This hard handover is used when the base station control station does not use the subscriber line extension technique, i.e., when the communication system does not have any interface connecting base station control stations with one another. In this case, different base station control stations are used before and after the transfer of the mobile device under control of the upper-layer node of the base station control stations. This control is also called “relocation”.
A network configuration in which a relocation is carried out is explained hereinafter with reference to FIG. 7. The system shown in FIG. 7 includes NodeBs 71 to 72, which are base stations, base station control stations (RNCs: Radio Network Controllers) 61 to 62, and a core network apparatus (CN: Core Network) 51, which is an upper-layer station. The NodeB 71 forms a cell 81 and the NodeB 72 forms a cell 82.
The RNC 61 is assigned with an RNC ID (RNC ID_61). Further, the RNC 62 is assigned with an RNC ID (RNC ID_62). When a mobile device moves from the cell 81 to the cell 82, the RNC 61 identifies the cell at the transfer destination as the cell 82 and also identifies the RNC 62 that is managing the transfer destination cell according to information notified from the mobile device. The RNC 61 notifies the CN 51 that the mobile device has moved to the network managed by the RNC 62. Then, the CN 51 establishes a data communication path between the mobile device and the RNC 62, thereby completing the transfer of the mobile device.
When communication using femto cells is carried out, there is no I/F between femto base stations and between femto base station control stations. Therefore, in order to carry out the mobility control, it is necessary to carry out a relocation (hard handover) process. Accordingly, as femto cells have become more widespread, the demand for efficient relocation processes is growing.
Patent literature 1 discloses a transfer method from a femto cell in which communication is permitted to a femto cell in which communication is not permitted. In this case, the base station control station that manages the femto cell in which communication is not permitted refuses the handover and the mobile device, which is refused the handover, continues to communicate to the limit of the reception power level in the currently-located femto cell.